The present invention relates to sample digestion. More particularly, the invention relates to the combined use of temperature and ultraviolet radiation to digest a sample prior to analysis.
Many chemical and biological assays and treatments require both heating a sample and exposing the sample to ultraviolet light to alter the sample constituents into an acceptable form for the desired application.
Previous digesters that require both heating and ultraviolet exposure have relied upon either of the following methods. In the first method, the sample is held at an elevated temperature for an extended period of time either before or after being moved to an ultraviolet digester for an extended period of time; see, e.g., scientific journal articles by Fernando et al. and by Benson et al., discussed below. This method requires the additional time associated with performing thermal and ultraviolet digestion separately and sequentially.
In the second method, the sample is placed in intimate contact with the ultraviolet source, and the sample comes to the same temperature as the source; see, e.g., U.S. Pat. Nos. 7,391,041, 7,993,580, and 8,377,375, and Publication JP 2011181266, discussed below. The disadvantage of this method is that the source and the sample must be at or near the same temperature. Often the temperature of a sample must be raised significantly in order to obtain the desired effects of thermal digestion. If the mercury ultraviolet source is allowed to reach these higher temperatures, the efficiency of the source will decrease due to self-absorption of mercury emission by volatilized mercury, thereby resulting in smaller contributions from the ultraviolet (UV) digestion.
U.S. Pat. No. 7,391,041 discloses means for heating a UV lamp to enhance emitting efficiency of the lamp. It is apparent that the means chosen, in which the heating means are applied directly to the envelope or envelopes surrounding the UV lamp would, in the case of the outer envelope, heat the sample fluid in contact with the outer envelope. Furthermore, heating the sample fluid would necessarily heat the UV lamp, and heating the UV lamp would necessarily heat the sample fluid.
U.S. Pat. Nos. 7,993,580 and 8,377,375 disclose the use of thermostatted lamps. However, this arrangement relates only to the temperature of the lamp, not to the temperature of the sample.
Publication JP 2011181266 teaches controlling lamp temperature by use of heat-resistant plates and by varying the volume of wind through the apparatus. As in the case of the two patents just discussed above, the only feature of the apparatus that is affected is the temperature of the lamp.
Fernandes, Lima, and Rangel [J. Anal. Chem (2000) 366: 112-115] describe a method using a two-stage photooxidation/thermal-digestion procedure in which polyphosphates are hydrolyzed with acid and heat, and organophosphates are digested by UV-catalyzed peroxydisulfate oxidation. Clearly, this stagewise procedure requires sequential time expenditures.
Benson, McKelvie, Hart, Truong, and Hamilton [Analytica Chimica Acta 326 (1996) 29-39] describe a method using UV/thermal-induced digestion by means of a UV photoreactor and a thermal-digestion unit connected in series. Clearly, this method likewise suffers the disadvantage of sequential time expenditures.